The IEEE 488 General Purpose Interface Bus (GPIB) is an international instrumentation interface standard for integrating instruments, computers, printers, plotters, and other measurement devices into systems. IEEE stands for the Institute of Electrical and Electronics Engineers and is currently the world’s largest technical professional society. Refer to http://www.ieee.org for more information about the society and its standards.

The interface between the VectorStar VNA and other devices on the GPIB is via a standard 24-wire GPIB interface cable. This cable uses a double-sided connector where one connector face is a plug and the other a receptacle. These double-function connectors allow parallel connection of two or more cables to a single instrument connector.

To achieve design performance on the GPIB network bus, the proper timing and voltage level relationships must be maintained. If either the cable length between separate instruments or the accumulated cable length between all instruments is too long, the data and control lines cannot be driven properly and the system may fail to perform.

The devices on the GPIB are connected in parallel, as shown in Figure: GPIB Interface Connection and Bus Structure. The interface consists of 16 signal lines and 8 ground lines in a shielded cable. Eight of the signal lines are the data lines, DIO 1 through DIO 8. These data lines carry messages (data and commands), one byte at a time, among the GPIB devices. Three of the remaining lines are the handshake lines that control the transfer of message bytes between devices. The five remaining signal lines are referred to as interface management lines.

The following paragraphs provide an overview of the GPIB including descriptions of:

Effective communications between devices on the GPIB requires three functional elements; a talker, a listener, and a controller. Each device on the GPIB is categorized as one of these elements depending on its current interface function and capabilities.

The GPIB uses 16 signal lines to carry data and commands between the devices connected to the bus. The interface signal lines are organized into three functional groups.

The signal lines in each of the three groups are designated according to function. Table: Interface Bus Signal Line Designations lists these designations.

The data bus is the conduit for the transfer of data and commands between the devices on the GPIB. It contains eight bidirectional, active-low signal lines —DIO 1 through DIO 8. Data and commands are transferred over the data bus in byte-serial, bit-parallel form. This means that one byte of data (eight bits) is transferred over the bus at a time. DIO 1 represents the least-significant bit (LSB) in this byte and DIO 8 represents the most-significant bit (MSB). Bytes of data are normally formatted in seven-bit ASCII (American Standard Code for Information Interchange) code. The eighth (parity) bit is not used.

Each byte placed on the data bus represents either a command or a data byte. If the Attention (ATN) interface management line is TRUE while the data is transferred, then the data bus is carrying a bus command which is to be received by every GPIB device. If ATN is FALSE, then a data byte is being transferred and only the active listeners will receive that byte.

Control of the transfer of each byte of data on the data bus is accomplished by a technique called the “three-wire handshake”, which involves the three signal lines of the Data Byte Transfer Control Bus. This technique forces data transfers at the speed of the slowest listener, which ensures data integrity in multiple listener transfers. One line (DAV) is controlled by the talker, while the other two (NRFD and NDAC) are wired-OR lines shared by all active listeners. The handshake lines, like the other GPIB lines, are active low. The technique is described briefly in the following paragraphs and is depicted in Figure: Typical GPIB Handshake Operation. For further information, refer to the ANSI/IEEE Standard 488.1 specification.

The general interface management bus is a group of five signal lines used to manage the flow of information across the GPIB. A description of the function of each of the individual control lines is provided below.

An interface function is the GPIB system element which provides the basic operational facility through which a device can receive, process, and send messages. Each specific interface function may only send or receive a limited set of messages within particular classes of messages. As a result, a set of interface functions is necessary to achieve complete communications among devices on the GPIB.

The ANSI/IEEE Standard 488.1 specification defines each of the interface functions along with its specific protocol. The ANSI/IEEE Standard 488.2 specification specifies the minimum set of IEEE 488.1 interface capabilities that each GPIB device must have. This minimum set of interface functions assures that the device is able to send and receive data, request service, and respond to a device clear message. Table: VectorStar VNA Interface Function Capability lists the interface function capability of the VectorStar Series VNA.

There are three types of information transmitted over the GPIB:

The controller manages the flow of information on the GPIB using interface function messages, usually called commands or command messages. Interface function messages perform such functions as initializing the bus, addressing and unaddressing devices, and setting device modes for remote or local operation.

There are two types of commands: multiline and uniline. Multiline commands are bytes sent by the active controller over the data bus (DIO1-DIO8) with ATN set TRUE. Uniline commands are signals carried by the individual interface management lines.

The user generally has control over these commands; however, the extent of user control depends on the implementation and varies with the specific GPIB interface hardware and software used with the external controller.

These commands are keywords or mnemonic codes sent by the external controller to control the setup and operation of the addressed device or instrument. The commands are normally unique to a particular instrument or class of instruments and are described in its documentation. Device-specific commands are transmitted over the data bus of the GPIB to the device in the form of ASCII strings containing one or more keywords or codes. They are decoded by the device’s internal controller and cause the various instrument functions to be performed.

These messages are sent by the device to the external controller via the GPIB. They contain measurement results, instrument status, or data files that the device transmits over the data bus in response to specific requests from the external controller. The contents of these messages are instrument specific and may be in the form of ASCII strings or binary data. In some cases data messages will be transmitted from the external controller to the device. For example, messages to load calibration data.

An SRQ (service request) is an interface function message sent from the device to the external controller to request service from the controller, usually due to some predetermined status condition or error. To send this message, the device sets the SRQ line of the General Interface Management Bus true, then sends a status byte on the data bus lines.

An SRQ interface function message is also sent by the device in response to a serial poll message from the controller, or upon receiving an Output Status Byte(s) command from the controller. The protocols associated with the SRQ functions are defined in the ANSI/IEEE Std 488.2 document.

The manner in which interface function messages and device-specific commands are invoked in programs is implementation specific for the GPIB interface used with the external controller. Even though both message types are represented by mnemonics, they are implemented and used in different ways.

Normally, the interface function messages are sent automatically by the GPIB driver software in response to invocation of a software function. For example, to send the IFC (Interface Clear) interface function message, one would call the ibsic function of the National Instruments software driver. On the other hand, the command *RST (Reset) is sent in a command string to the addressed device. In the case of the National Instruments example, this would be done by using the ibwrt function call.

Table: VectorStar VNA Response to GPIB Interface Function Messages lists the GPIB interface function messages that the MS464xB will recognize and respond to. With the exception of the Device Clear and Selected Device Clear messages, these messages affect only the operation of the VectorStar VNA GPIB interface. The VNA response for each message is indicated below. Interface function messages are transmitted on the GPIB data lines and interface management lines as either unaddressed or addressed to receive the commands. The manner in which these messages are invoked in programs is implementation dependent. For more programming information, refer to the documentation included with the GPIB interface used for the external controller and to the IEEE 488.1 specification.

Use this procedure to setup the dedicated GPIB port to control other GPIB devices such as Power Meters or Signal Generators by using the VNA rear panel Dedicated GPIB Port. An example of a complex GPIB configuration with the VNA as the GPIB controller is shown in Figure: VNA as a Remote GPIB Controller for Synthesizers.

The GPIB address for any controlled device can be kept at the factory default values or changed as required. The default GPIB addresses are:

The GPIB addresses for the External Sources can be changed from the Ext Src Addr sub-menu available at:

Use this procedure to set up the talker/listener port to control the VNA remotely over an IEEE 488.2 GPIB bus network. In this configuration, an external computer/controller is physically connected to the VNA rear panel IEEE 488.2 GPIB Port with a standard GPIB cable. Most of the VectorStar VNA functions (except power on/off and initialization of the hard disk) can be controlled remotely by an external computer/controller via the IEEE-488.2 GPIB bus network.

Once the software has finished loading and start-up testing is complete, the instrument is ready to be remotely controlled via the GPIB. The instrument will not respond to GPIB commands until the instrument’s software has been loaded.